Mesa type combined direct viewing storage target and fluorescent screen for cathode ray tube



Sept. 10, 1968 R. w. MORRIS 3,401,293

MESA TYPE COMBINED DIRECT VIEWING STORAGE TARGET AND FLUORESCENT SCREEN FOR CATHODE RAY TUBE Filed Nov. 28, 1966 ROBERT W. MORRIS lA/VE/VTOR BUCKHORN, BLO/PE, KLAROU/ST a SPAR/(MAN ATTORNEYS 3,401,293 MESA TYPE COMBINED DIRECT VIEWING STOR- AGE TARGET AND FLUORESCENT SCREEN FOR CATHODE RAY TUBE Robert W. Morris, Portland, reg., assignor to Tektronix, Inc., Beaverton, 0reg., a corporation of Oregon Filed Nov. 28, 1966, Ser. No. 597,465 8 Claims. (Cl. 313-68) ABSTRACT OF THE DISCLOSURE An electron image storage tube capable of displaying both stored and nonstored charge images simultaneously in the same general area, is described, employing a combined display structure including a direct viewing storage target and a separate fluorescent screen provided on a common support member. The combined display structure includes a storage dielectric phosphor and a fluorescent screen phosphor provided on dilferent portions of the support member and intermixed but physically separated and longitudinally displaced from each other to provide a mesa-like display structure. A light reflecting metal layer is provided over the fluorescent screen phosphor Which prevents contamination of the fluorescent screen and increases the brightness of the light image emitted by such screen as well as acting as a collector electrode from the secondary electrons emitted by the storage dielectric phospor.

The subject matter of the present invention relates generally to electron discharge display devices, and specifically to cathode ray tubes which are capable of operating simultaneously both as a direct viewing bistable storage device and as a conventional nonstorage display device. The cathode ray tube of the present invention employs a mesa type of combined direct viewing storage target and fluorescent screen display structure to display both a stored and a nonstored signal image simultaneously on the same general area.

The present cathode ray tube employing a combined storage target and fluorescent screen display structure may be employed in a cathode ray oscilloscope. The combined storage target and fluorescent screen display structure of the present invention emits a light image of higher brightness than that of previous phosphor storage targets. The increase in brightness is believed to be achieved because the fluorescent screen is laterally displaced and physically separated from the storage dielectric so that such fluorescent screen is elfectively isolated from any source of foreign material which might contaminate the fluorescent screen and reduce its brightness. This contamination and resulting reduction in brightness is believed to be caused by material migrating from the storage dielectric and poisoning the fluorescent screen, which occurs when the combined display structure is heated to remove the light sensitive material mixed with the storage dielectric phosphor to photographically form a dot pattern, due to the chromic oxide residue left by such light sensitive material. The isolation of the fluorescent screen in the preferred embodiment of the present structure is accomplished by etching a glass support plate to provide interconnected cavities and spaced mesa portions, filling the cavities with the fluorescent screen phosphor material, applying protective layers of lacquer and electron transparent light reflecting metal over such fluorescent screen and then forming the storage dielectric of phosphor material on such protective layers. The result is an improved combined fluorescent screen and storage target display strucnited States Patent ture in which the storage-dielectric dots are supported on the mesa portions of the glass plate over the metal coated fluorescent screen laterally displaced from such storage dielectric.

A cathode ray tube made in accordance with the present invention is capable of bistable storage of a charge image for an indefinite controllable time, typically on the order of one hour, by bombarding the storage dielectric with low velocityflood electrons in a conventional manner to cause secondary electron emission from the storage dielectric, such secondary electrons being collected by the metal coating provided over the fluorescent screen. Thus this metal coating acts both as a light reflector for the light emitted by the fluorescent screen and as a collector electrode for secondary electrons emitted by the storage dielectric. Since the storage dielectric dots of phosphor material are uniformly distributed across the support plate of the display structure, such storage dielectric dots are intermixed with portions of the fluorescent screen to provide an array of alternate storage and nonstorage regions of phosphor material. As a result, both a stored electron image and a nonstored electron image can be displayed simultaneously on the same general area of the display structure.

In addition, the voltage on the collector electrode layer can be adjusted so that the storage dielectric dots are incapable of bistable storage until the desired waveform is displayed on the adjacent areas of fluorescent screen, and then such voltage increased to enable storage of such desired waveform. This enables previewing of a signal waveform before causing such waveform to be stored. However, the light image produced by the display structure of the present invention is much brighter due to the presence of a separate nonstorage fluorescent screen and the use of a light reflecting metal coating over such screen. It should be noted that the light reflecting metal layer is sufliciently thin to be penetrated by the writing beam of high velocity electrons but cannot be penetrated by the low velocity flood electrons or by the secondary electrons emitted from the storage dielectric so that there is no undesirable background lighting of the fluorescent screen.

-It is therefore one object of the present invention to provide an improved combined fluorescent screen and direct viewing storage target display structure which can produce a light image of both a stored electron image and a nonstored electron image simultaneously on the same display area, such nonstored image being of high brightness.

Another object of the present invention is to provide a cathode ray tube having a combined fluorescent screen and direct viewing bistable storage target display structure of simple, rugged and inexpensive construction.

A further object of the present invention is to provide an improved, combined fluorescent screen and direct viewing bistable storage target display structure in which the fluorescent screen is laterally displaced and physically separated from the storage dielectric to effectively isolate the fluorescent screen and prevent its contamination by foreign material which tends to reduce the brightness of the light image emitted by such fluorescent screen.

An additional object of the present invention is to provide an improved cathode ray tube which is capable of bistable storage operation as well as conventional nonstorage fluorescent screen display operation simultaneously without reducing the brightness of the nonstored image or otherwise penalizing the performance of the cathode ray tube in its nonstorage operation.

Other objects and advantages of the present invention will be apparent from the following detailed description 3 of a preferred embodiment thereof and from the attached drawings of which:

FIG. 1 is a plan elevation view of a section of a cathode ray tube made in accordance with the present invention to include a combined storage target and fluorescent screen display structure provided on the face plate of such tube;

FIG. 2 is an enlarged horizontal section view taken along the line 22 of FIG. 1, which includes a schematic representation of the electron guns employed in the tube;

FIGS. 3A, 3B, 3C, 3D and 3E are schematic diagrams indicating different steps in the method of manufacture of the combined display structure shown in FIGS. 1 and 2; and

FIG. 4 is an enlarged section view of another embodiment of the combined display structure which is similar to that of FIG. 2.

As shown in FIGS. 1 and 2, the cathode ray tube of the present invention has a glass face plate 12 secured to a funnel shaped envelope portion 14 of ceramic or glass material by a glass frit seal 16 to provide the evacuated envelope of such tube. The inner surface of the glass face plate 12 is etched to provide a plurality of interconnected cavities 18 within which is deposited a fluorescent scren 20 of P31 or other sulfide type phosphor material having high brightness and nonstorage characteristics. A light reflecting layer 22 of evaporated metal, such as aluminum or gold, is provided over the surface of the fluorescent screen 20 in order to increase the brightness of the light image emitted by such fluorescent screen in a conventional manner. A storage dielectric 24 in the form of a plurality of spaced dots of P-l or other high dielectric silicate type phophor material capable of bistable storage, are provided on the tops of spaced mesa portions 26 of the glass face plate 22 which are formed a during etching. As a result, the storage dielectric dots 24 are displaced longitudinally of the tube and physically separated from the fluorescent screen 20 by the metal layer 22 and by lacquer layers (not shown) which are provided on opposite sides of such metal layer but are removed during a heating step in the method of manufacture as hereafter described. Thus the metal layer 22 is provided over the fluorescent screen 20 but not over the glass mesa portions 26, so that the light emitted from the phosphor storage dielectric 24 can be transmitted through the face plate 12 to the viewer. The resulting combined display structure includes storage dielectric phosphor areas 24 and fluorescent screen phosphor areas 20 provided on different portions of the support member 12 but intermixed to enable simultaneous displays of stored and nonstored charge images on the same general 0 area of the display structure.

In addition to functioning as a light reflector, the metal layer 22 also acts as a collector electrode for the secondary electrons emitted from the phosphor storage dielectric 24 during storage when such storage dielectric is bombarded by low velocity flood electrons. The metal layer 22 is connected to a source of positive DC. bias voltage located external to the tube by means of a thin transparent conductive =film 28 of tin oxide coated on the inner surface of the glass face plate beneath the glass frit seal 16. The conductive film 28 is electrically connected to the metal coating 22 by the connector layer 30 of evaporated aluminum or other suitable material within the cathode ray tube, and is electrically connected to the common connection of a pair of resistors 32 and 34 forming a voltage divider positioned outside of such tube. The opposite ends of the voltage divider formed by resistors 32 and 34 are connected to a source of positive D.C. supply voltage of +300 volts and to ground, respectively. Resistor 32 may be of a variable resistance in order to adjust the voltage applied to the tin oxide film 28. In order to enable bistable storage, the DC. bias voltage applied to the metal layer 22 which functions as the collector electrode is set at about +200 volts when the flood gun cathode is grounded and the writing gun cathode is connected to a negative DC. voltage of about 3000 volts.

As in conventional bistable storage tubes, a writing electron gun 36 provides a narrow beam of high velocity electrons which are deflected across the storage dielectric 24 and the fluorescent screen 20 to form charge images on these elements and to cause them to emit light images corresponding to such charge images. In addition, a flood electron gun 38 directs a wide beam of low velocity flood electrons uniformly over the entire surface of the display structure, so that such low velocity electrons strike the storage dielectric 24 and the metal layer 22. The metal layer 22 prevents the low velocity flood electrons from reaching the fluorescent screen 20 so that undesirable background illumination is eliminated. However the light reflecting metal layer 22 is sufliciently thin that it is transparent to the high velocity writing electrons which penetrate through such metal layer to excite the fluorescent screen 20. Likewise the secondary electrons emitted from the storage dielectric 24 by bombardment of such storage dielectric with the low velocity flood electrons are not of suflicient velocity to penetrate through metal layer 22 but are merely collected by such layer.

This bombardment of the storage dielectric dots 24 with the low velocity flood electrons causes bistable storage of any charge image formed thereon by the writing beam if the potential of such charge image is above the first crossover voltage of the secondary emission characteristic of the storage dielectric. The phosphor storage dielectric emits a light image corresponding to the stored charge image, to provide a direct visual readout of the stored information. In addition, if so desired, an electrical readout signal can be produced on the metal layer 22 which is transmitted to a'readout terminal 40 connected to the junction of resistors 32 and 34 by causing the writing gun 36 to scan the storage charge image with a reading beam of intermediate velocity electrons in a TV raster pattern so that such readout signal corresponds to the stored charge image. The resulting electrical readout signal can be supplied to a remote TV monitor to vary the brightness of the electron beam in such monitor as such beam is deflected in synchronism with the readout beam in the storage tube in order to produce a television display of the stored charge image.

One method of manufacture of the mesa type combined fluorescent screen and storage target display structure of the present invention is illustrated in FIGS. 3A to 3B. First the flat glass support plate 12 is etched on one side to provide cavities 18 and mesa portions 26, as shown in FIG. 3A. Any suitable etching technique can be employed. For example, after the plates are cleaned by scrubbing them with detergent, rinsing in water and then soaking in a solution of sodium dichromate and sulfuric acid heated to about C. for 60 minutes followed by additional rinsing and drying steps, a photosensitive resist coating is applied to the entire surface of the glass plate. Then the resist coated plate is exposed to light, except in those areas over the cavity portions 18, through a film negative or other light mask provided on the etched surface of the plate. After an exposure time of about one minute, the resist is removed from the unexposed areas overlying the cavity portions 18 to develop the image by immersing the plate in a suitable solvent. Then the resist coated glass plate 12 is placed in an etching solution of hydrogen fluoride. A 48 percent solution of hydrogen fluoride provides an etch of approximately .001 inch of depth per minute and the total depth of the etched cavities 18 is about .002 inch. Prior to etching, the back surface and the edges of the glass plate 12 may be further masked with a layer of asphaltum to prevent etching on these surfaces.

After the etching process is complete the photoresist and other masking material is removed. Then the conductive film 28 of tin oxide is applied as a ring around the outer edge of the etched side of the face plate by vapor deposition in a conventional manner, when such conductive film is employed to make contact with the metal layer 22. Then the etched cavity portions 18 of the glass plate 12 are filled with a water suspension of P-3l type phosphor material by means of a squeegee to form the fluorescent screen layer 20 of FIG. 3A. Next while the phosphor layer 20 is still wet, a lacquer layer 42 is sprayed over the outer surface of such phosphor and the exposed tops of support portions 26 of the glass plate, as shown in FIG. 3B, in order to provide a smooth surface for the light reflecting metal layer 22, which then is evaporated over such lacquer layer.

As shown in FIG. 30, the portions of the lacquer layer 42 and the evaporated aluminum layer 22 on top of the mesas 26, are removed by scraping or polishing. The re maining portions of the lacquer layer 42 and the metal layer 22 overlying the fluorescent screen phosphor 20 are not removed because they shrink and sag below the outer surface of the glass mesas 26 during drying.

As shown in FIG. 3D, a second lacquer layer 44 is provided over the outer surface of the metal layer 22 and the tops of the mesas 26 to seal any breaks in such metal around the mesas 26 which may be caused during the scraping, and to make sure that the fluorescent screen phosphor 20 is completely isolated from the storage dielectric layer or any other possible source of foreign material which might contaminate such fluorescent screen and reduce its brightness. Then a layer 46 of P-l type phosphor and photosensitive solution including dichromate sensitized polyvinyl alcohol, is applied over the second lacquer layer 44. A water suspension of grams of P-l phosphor material to 100 millilitres of photosensitive solution is employed, such photosensitive solution having the proportions of 100 grams of polyvinyl alcohol to 1000 millilitres of Water and 1000 millilitres of ethanol. The polyvinyl alcohol solution is photosensitized by adding about .14 gram of ammonium dichromate solution per 100 millilitres of polyvinyl alcohol solution to produce the photosensitive solution.

The layer 46 of phosphor material and photosensitive material is then exposed to light emitted from source 48 positioned on the opposite side of the glass plate 12, so that such light is transmitted only through the glass mesas 26 to expose the portions of layer 46 above such mesas. The light is prevented from reaching the portions of layer 46 above the fluorescent screen phosphor by the light reflecting metal layer 22. After exposure for about 14 minutes, the unexposed portions of the photosensitized layer 42 are removed by washing with Water, leaving only the exposed hardened dots above the glass mesas 26.

As shown in FIG. 3E the coated plate 12 is then heated by an electric heating element 50 in an oven for minutes at about 410 C. to remove the lacquer coatings 42 and 44 and to remove the photosensitive material in the remaining dot portions of the layer 46 to form the phosphor dots 24 which make up the storage dielectric of the tube of FIGS. 1 and 2. Finally, any phosphor material overlying the tin oxide film 28 or the gap between such film and the edge of metal layer 22, is removed and the connecting ring layer 30 of aluminum is evaporated on the glass plate to electrically connect the tin oxide film 28 to the metal layer 22.

Since no photosensitive solution is employed in applying the fluorescent screen phosphor 20 and such fluorescent screen is isolated from the photosensitive material employed in the formation of the storage dielectric phosphor dots 24 by the lacquer layers 42 and 44 as well as the aluminum layer 22, no dichromate poisoning of such fluorescent screen occurs, which might otherwise greatly reduce the brightness of such fluorescent screen. After the combined display structure is completed, it is attached to the ceramic funnel portion 14 of the cathode ray tube envelope by a glass frit seal 16 to form an envelope.

Another embodiment of the combined storage target and fluorescent screen display structure of the present invention is shown in FIG. 4. This display structure is similar to that of FIG. 2 so that the same reference numbers have been used to indicate the corresponding elements in FIG. 4. In addition a thin, light transparent conductive coating 52 of tin oxide or other suitable material is provided on the etched surface of the glass face plate 12 beneath the storage phosphor 24 and the fluorescent screen phosphor 20. Such tin oxide coating may also extend through the glass frit seal 16 in order to be connected to a source of collector voltage in the manner of the conductive film 28 of FIG. 2. The combined display structure of FIG. 4 has been found to be extremely useful when a writing beam of high energy above about 6 kilovolts is employed, such as in a cathode ray tube having post de flection acceleration of the writing beam. When the combined display structure of FIG. 2 is employed with writing beams of very high energy the stored writing speed of the tube decreases. This is apparently due to the fact that negative charge produced on the fluorescent screen phosphor 20 by the writing beam, penetrates through the aluminum collector electrode 22 which is relatively porous to such field. This negative field remains for a short time following the path of the writing beam across the fluorescent screen and the charge produced on the fluorescent screen by a high energy writing beam requires a longer time to leak off to the collector electrode. Apparently the resulting negative field causes inefficient collection by collector electrode 22 of the low velocity secondary electrons emitted from the storage phosphor 24 during bombardment by the flood electrons. The repelling of the secondary electrons away from the collector electron by the negative field tends to prevent bistable storage of any charge image formed on the storage phosphor by the writing beam and therefore reduces the stored writing speed. This problem is corrected by the display structure of FIG. 4 because the portions of the tin oxide coating 52 beneath the storage phosphor 24 also acts as a collector electrode which, however, is not affected by the negative field of the fluorescent screen. Also the portions of the tin oxide coating 52 beneath the fluorescent screen phosphor 20 enable the negative charge produced on such fluorescent screen to leak off faster.

It will be obvious to those having ordinary skill in the art that many changes may be made in the above-described details of the preferred embodiment of the present invention without departing from the spirit of the invention. Therefore the scope of the present invention should only be determined by the following claims.

I claim:

1. An electron image storage tube having means for forming a charge image on a storage target with said tube and means for bombarding the storage target with low velocity electrons to cause bistable storage of said charge image, the improvement comprising:

a combined display structure including a direct viewing storage target and fluorescent screen, said display structure including a storage dielectric of phosphor material and a fluorescent screen of phosphor material provided on different portions of a common support member with the storage dielectric portions and fluorescent screen portions intermixed but physically separated and displaced from each other longitudinally of the tube to provide a mesa-like display structure, said storage dielectric being capable of bistable storage of a charge image formed thereon and emitting a light image corresponding to the stored charge image and said fluorescent screen emitting a light image but being incapable of bistable storage under the same conditions,

isolation means including portions of said support member provided between the storage dielectric and the fluorescent screen, for isolating said storage dielectric from said fluorescent screen to prevent contamination of said fluorescent screen which would reduce its brightness.

2. A storage tube in accordance with claim 1 in Which the combined structure includes:

a support member in the form of a plate of light transparent insulating material having a plurality of intermixed cavity portions and land area portions provided on the outer surface of one side of said plate;

a fluorescent screen of phosphor material supported on said outer surface of said support member in said cavity portions;

a storage target including a storage dielectric of phosphor material secured to said support member on said land area portions so that the portions of the storage dielectric and fluorescent screen are intermixed on said outer surface of said support member with said storage dielectric positioned above said fluorescent screen; and

a light reflecting layer of conductive material provided over said fluorescent screen, such conductive layer being sufliciently thin to be transparent to high velocity electrons.

3. A storage tube in accordance with claim 2, which also includes a thin, light transparent film of conductive material coated over said cavity and land area portions of said support member beneath said fluorescent screen and said storage target, and means for applying a DC. bias voltage to said film and said conductive layer to enable them to collect secondary electrons emitted from the storage dielectric portions.

4. A storage tube in accordance with claim 2 in which the land area portions are separate and spaced from each other to provide the storage dielectric in the form of a plurality of spaced phosphor dots and the cavity portions are interconnected to provide the fluorescent screen in the form of an integral phosphor layer.

5. A storage tube in accordance with claim 2 in which the support member is the glass face plate of the tube and has its inner surface etched to provide the intermixed cavity portions and land area portions.

6. A storage tube in accordance with claim 1 in which the charge image forming means includes,

means for producing a narrow beam of high velocity electrons and for deflecting said beam across said fluorescent screen and said storage dielectric.

7. A storage tube in accordance with claim 1 in which the fluorescent screen and the storage dielectric are made of different phosphor materials.

8. A storage tube in accordance with claim 1 in which the fluorescent screen and the storage dielectric are made of different thicknesses.

References Cited UNITED STATES PATENTS 2,121,356 6/1938 Knoll 31368 2,155,465 4/1939 Behne et al. 313-68 2,833,948 5/1958 Evans 31368 3,293,474 12/1966 Gibson 31368 3,312,850 4/1967 McMillan et al 3l3-68 3,339,099 8/1967 Anderson 313-68 ROBERT SEGAL, Primary Examiner.

3 35 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,4 1, 293 Dated September 10, 1968 Inventofls) RwERT W. MORRIS It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

r- In the specification, column 3, line 25, "scren" should read --screen--.

In the claims, column 6, line 50, "with" should read -within--.

SIGNED AN'D SEALED FEB a 4970 (SEAL) Attest: Edwardm. Fletcher, Jr- WILLIAM E. soHuYLER, JR A tin Officer fisioner of Patents ttes g 

